Bifunctional lipase mutant and methods of using same

ABSTRACT

The present invention provides a novel dual-function lipase variant and its application in processing of flour products. The amino acid sequence of the lipase has one of the following amino acid substitutions: P298T, P298T/H317P, P298T/H317P/V326S, P298T/T218S/S234F, P298T/H317P/P168L/A129S, P298T/S234F/K161R/V326S or the nucleotide sequence of wherein said lipase is substituted, deleted, or added based on the sequence encoding the amino acid described in (a) and has at least 80% identity with it. The variants have good performance in processing of flour products, while they can significantly whiten the bread or other products in processing of flour products and significantly increase the specific volume in bread baking process.

FIELD OF THE INVENTION

The present invention relates to a novel dual-function lipase variantand its application in processing of flour product. Specially, thelipase variant with enhanced characteristics may be obtained bymolecular biology techniques, which is suitable for processing of flourproducts. The present invention relates to the field of enzymeengineering.

BACKGROUND

Enzyme plays an irreplaceable role in improvement of baking and flourquality because of its unique biological characteristics. Currently, theusage of enzymes in processing of flour products has become a mainstreamtrend.

The flour used for bread baking contains glycerol phospholipids andtriglycerides. When the glycerol phospholipids were partially hydrolyzedinto hemolysis glycerophospholipids and triglycerides were partiallyhydrolyzed into monoglycerides or diglycerides, the hemolysisglycerophospholipids, monoglycerides and/or diglycerides would work assurfactants during emulsion reaction. In addition, emulsification playsa part in strengthening the dough gluten, enhancing urgent expansion inthe furnace and expanding volume, which makes the bread uniform, softand have a better taste. Therefore, addition of enzyme which hashydrolysis activity of glycerol phospholipids and triglycerides to theflour during bread baking process would obtain effects as describedabove. Moreover, pigments such as lutein and carotene in flour wouldaffect the whiteness of flour products. If an enzyme which hashydrolysis activity of triglycerides is added to the flour to degradefat, the fat-soluble pigments would be released and then fade moreeasily through oxidization. As a result, the flour products would bewhitened.

In previous studies, the inventors successfully screened Rhizopuschinensis CCTCC M 201021 from Luzhou-flavor liquor brewing koji, whichhas a high yield of lipase and cloned the lipase gene from the genomefor the first time and then the recombinant strain achieved high levelsecretion for the lipase in Pichia pastoris (Yu Xiao-Wei et al J MolCatal B: Enzym, 2009, 57: 304-311). The inventors also used the lipasegene as the template and obtained a series of variants with improvedthermostability by directed evolution (Granted patent: CN 101974499B; YuXiao-Wei et al. Microbial Cell Factories, 2012, 11:102-112). Then, thelipase gene from R. chinensis was mutated by molecular biology in thisinvention and heat resistant variants with hydrolytic activity oftriglyceride and lecithin were obtained. The variants performed betterat whitening bread or other products during processing of flour productsand increasing the specific volume in bread baking process. As a grantedpatent CN 102160562 B has disclosed, the parent lipase as an additivehas improved the baking characteristics of bread baking, while thepresent invention provides more excellent lipase variants which could beused in processing of flour products.

DETAILED DESCRIPTION

The goal of the present invention is to provide a novel lipase variant,which has one of the characteristics described as follows:

(1) whose amino acid sequence has comprises one of the substitutionsrelative to the parent amino acid sequence shown in SEQ ID NO.1:

mutant 1: P298T (Proline→Threonine);

mutant 2: P298T/H317P (Proline→Threonine and Histidine→Proline);

mutant 3: P298T/H317P/V326S (Proline→Threonine, Histidine→Proline andValine→Serine);

mutant 4: P298T/T218S/S234F (Proline→Threonine, Threonine→Serine andSerine→Phenylalanine);

mutant 5: P298T/H317P/P168L/A129S (Proline→Threonine, Histidine→Proline,Proline→Leu and Alanine→Serine);

mutant 6: P298T/S234F/K161R/V326S (Proline→Threonine, Serine→Phe,Lysine→Argnine and Valine→Serine);

(2) by deletion, substitution, insertion or mutation of one or morebases, a nucleotide sequence of the lipase has at least 80% identitywith a nucleotide sequence encoding the amino acid sequence described in(1), and the lipase maintains a triglyceride and lecithin hydrolysisactivity and good performance in thermostability.

The lipase variant exhibits better thermostability, and its half-life isprolonged by 14-38 fold at 40° C. than the parent lipase.

The present invention also provides a genetic engineering strainharboring a sequence encoding the lipase variant and its construction.DNA sequence encoding the lipase variants and genetically engineeredbacteria or transgenic cell lines harboring the DNA sequence is also inthe true scope of the present invention.

The genetic engineering strain is P. pastoris GS115, KM71 or SMD1168,and preferred GS115.

The present invention also provides a method of constructing the geneticengineering strain. Expression vector pPIC9, pPIC3K, pPIC9K, pAO815 orpPICZα is selected to construct a recombinant expression vector and P.pastorisGS115 is selected as the host to express the lipase variant.More preferably, the expression vector is pPIC9.

Application of the above lipase variants in processing of flour productsalso belongs to the true scope of the present invention. The flourproducts comprise bread, steamed bread and noodles.

Nomenclature for amino acid modifications in the present invention isexplained in detail as follows.

The mutated amino acid in the variant is marked as “original amino acid,position, substituted amino acids”. For example, S234F indicates asubstitution of Ser in position 234 with Phe. The position numbercorresponds to the amino acid sequence of the parent lipase shown in SEQID NO: 1, and L180H/T128S indicates the position of 180 and 218 are bothmutated.

The lipase variants provided in the present invention are thermostableand have triglyceride hydrolysis activity and lecithin hydrolysisactivity. While they can significantly whiten the bread or otherproducts and significantly increase the specific volume in bread bakingprocess, the lipase variants have good application in processing offlour products.

EMBODIMENTS

Media:

LB liquid medium: 10 g·L⁻¹ trypton, 5 g·L⁻¹ yeast extract, 10 g·L⁻¹NaCl, pH7.0.

MD (Minimal dextrose medium): 13.4 g L⁻¹ YNB, 4×10⁻⁴ g L¹ biotin, 20g·L⁻¹ dextrose, 20 g·⁻¹ agar.

BMGY (Buffered glycerol-complex medium): 10 g·L⁻¹ yeast extract, 20g·L⁻¹ trypton, 13.4 g L⁻¹ YNB, 4×10⁻⁴ g L⁻¹ biotin, 10 g·L⁻¹ glycerol,pH 6.0 100 mM potassium phosphate.

BMMY (Buffered methanol-complex medium): 10 g·L⁻¹ yeast extract, 20g·L⁻¹ trypton, 13.4 g L⁻¹ YNB, 4×10⁻⁴ g L⁻¹ biotin, 5 g·L⁻¹ methanol,100 mM potassium phosphate.

Example 1 Construction of Recombinant Expression Vector by Site-DirectedMutagenesis

The pre-constructed plasmid pPIC9K-proRCL (Wang Lele et al. Cloning andexpression of pro- and mature Rhizopus chinensis lipase in Pichiapastoris. The high-tech communications, (2009), 19 (10): 105) containsthe lipase gene of the parent strain R. chinensis CCTCC M201021 and itwas used as a template. Site directed mutagenesis kit (QuikChange II XLSite-Directed Mutagenesis Kit, Agilent) was used to obtain the followingsix mutated plasmids:

mutant 1: P298T,

mutant 2: P298T/H317P,

mutant 3: P298T/H317P/V326S,

mutant 4: P298T/T218S/S234F,

mutant 5: P298T/H317P/P168L/A129S,

mutant 6: P298T/S234F/K161R/V326S.

Example 2 Construction of Recombinant Strain Expressing the LipaseVariant

The mutated plasmid was transformed into competent cells by heat shock.The positive strains were then selected from the LB media containingampicillin. And then the plasmids were extracted from the strains andsequenced for verification.

The extracted correct mutated plasmid was digested with restrictionendonuclease Sal I and then collected and condensed. Then, thelinearised plasmid was mixed with 80 μL competent P. pastoris GS115 andremoved to 0.2 cm cuvette for electric shock. The shock was conducted atvoltage of 1500 V with the capacitance of 25 μF and the resistance of200Ω.

After the shock, 1 mL pre-icecold sorbitol solution with a concentrationof 1 mol/L was immediately added and mixed. Then the mixture wasstanding at 30° C. for 1 h before applying on MD plate to screening thecorrected recombinant which integrated the target gene to itschromosome. The yeast genome was extracted and amplified by primers5′AOX and 3′AOX for PCR identification. As a negative control, thestrain contained the empty vector was used to validate that the targetgene had been integrated into the yeast genome.

Example 3 Expression and Secretion of the Lipase Variant and itsSeparation and Purification

The positive recombinant yeast was cultivated in 25-50 mL of BMGY mediumin 250 mL glass flasks. When cultures reached an OD of 2.0-6.0, thecells were centrifuged and resuspended with 25-50 mL of BMMY mediumshaken at 20-30° C. and 100-250 rpm for 72-144h. Methanol was added withan amount of 1% of the broth volume every 24 h.

Protein separation and purification was achieved by the following steps.

(1) Concentrated by 10 KD Ultrafiltration

100 mL broth was centrifuged at 4° C. and 4000 rpm for 20 min. Then thesupernatant was filtered by 0.22 μm microporous membrane andconcentrated to about 10 mL through an 10 KD ultrafiltration. Theconcentrated enzyme solution was interchanged overnight with 0.02 mol/LHAc-NaAc buffer (pH 5.0) at 4° C.

(2) Purified by Strong Cation Exchange Chromatography

The interchanged solution was loaded onto a pre-equilibratedSP-Sepharose FF column (1.6 cm×20 cm) and eluted with the same buffer(0.02 mol/L pH 5.0 HAc-NaAc buffer) to remove the unadsorbed protein.Then the absorded protein was eluted with 0-0.5 M NaCl in the samebuffer at a flow rate of 1 mL/min. The lipase activity component wasconcentrated stepwise and interchanged overnight with 0.05 M potassiumphosphate buffer (pH7.5) containing 1.6 M ammonium sulfate at 4° C. foruse.

(3) Purified by Hydrophobic Interaction Chromatography

The interchanged enzyme solution of step 2 was then treated with aPhenyl-sepharose 6 FF column (1.6 cm×20 cm). 0.05 M potassium phosphatebuffer (pH7.5) containing 1.6 M ammonium sulfate was used as theequilibration buffer and also used to elute the unadsorbed protein.Lipase was then eluted by an ammonium sulfate concentration gradient of0.4 M in 0.05 M potassium phosphate buffer (pH7.5) and then eluted withwater at a flow rate of 0.8 mL/min. And 4 mL fractions containing theactivity component were collected, interchanged and then freeze-driedfor use.

Example 4 Lipase Properties

(1) Hydrolysis Activity

(a) Determination of Triglyceride Hydrolysis Activity

10 mL water was added to 200 mg olive oil and 100 mg arabic gum. Themixture was then emulsified by stirring at 10000 rpm for 5 min. Next,The 200 μL emulsified solution, 100 μL 0.2 M MOPS buffer (pH 6) and 20μl 0.1 M calcium chloride solution was mixed and held at 40° C. for 5min. Then 40 μL enzyme solution was added, mixed and held at 40° C. for5 min before 40 μL hydrochloric acid (1N) was added to terminate theenzymatic reaction. Finally, 400 μL 4% Triton X 100TRITON-X-100 wasadded to the mixture to release the free fatty acids.

(b) Determination of Lecithin Hydrolysis Activity

In this case, lecithin was used as a substrate. At first, 10 mL waterwas added to 200 mg lecithin and 400 μL TRITON-X-100. The mixture wasthen emulsified by stirring at 10000 rpm for 5 min. Next, the 500 μLemulsified solution, 250 μL 0.2 M MOPS buffer (pH 6) and 20 μL 0.1 Mcalcium chloride solution was mixed and held at 40° C. for 5 min. Andthen 40 μL enzyme solution was added, mixed and held at 40° C. for 10min before 100 μL hydrochloric acid (1N) was added to terminate theenzymatic reaction. Finally, 400 μL 4% TRITON-X-100 was added to themixture to release the free fatty acids.

The method of quantifying the free fatty acids was as follows. 3 mL freefatty acid quantitative reagent NEFA was added to 30 μL the abovereaction mixture and then kept at 40° C. for 10 min.

One unit of enzyme activity was defined as the amount needed to release1 μmol fatty acid per minute under 40° C. and pH 6.0.

The results were shown in Table 1. The ratio of the triglyceridehydrolysis activity and lecithin hydrolysis activity of the purifiedlipase variants were decreased from 4.9 to 1.5-3.2, indicating that thelecithin hydrolysis activity of the variants was increased compared tothe triglyceride hydrolysis activity.

(2) Thermal Stability

The determination of half-life of the enzyme at 40° C. was carried outby the following steps. The enzyme solution was treated in 40° C. atvarious time intervals and the percentage (%) of residual triglyceridehydrolysis activity was determined. The time (min) was set as the X-axisand the logarithm of the percentage (%) of residual activity was set asthe Y-axis. The inactivation constant kinact k_(inact) was determined bythe slope and the half-life of the enzyme (t₅₀) at 40° C. was determinedby t₅₀=ln 2/k_(inact).

Results of thermal stability (Table 1) showed that the half-life of thevariants at 40° C. were prolonged by 14-38 fold compared with the parentlipase.

TABLE 1 Enzymatic properties of lipase Ratio of triglyceride Prolongedfold of hydrolysis activity and leci- thermal stability thin hydrolysisactivity compared to the Enzyme (U/mg:U/mg) parent lipase the parentlipase 4.9 — mutant 1 3.2 14 mutant 2 2.7 25 mutant 3 1.8 18 mutant 42.9 29 mutant 5 2.2 38 mutant 6 1.5 27

Example 5 Application of Lipase Variants in Bread Baking

The bread baking experiments mainly focused on the impact of the lipaseon the bread specific volume, and the application effect was comparedbetween the parent lipase and the variants.

The preparation of bread was referenced to the AACC 10-10B method with afew modification. 100% flour, 1% salt, 4% sugar, 4% butter, 1.5% yeastand 62.5% water (according to the wheat mass) were mixed and lipase wasadded to it with an amount of 500 U/kg, while the amount of lipase inthe blank control was zero. The mixture were stirred in a stirrer for 10min, and then stood for 10 min before divided into 100 g. Next, themixture was rounded, and stood for 10 min again. The dough was formedand put on plates and then proofed at 38° C. with a relative humidity of85% for 90 min. Finally, the dough was baked for 25 min (upper-sidetemperature was 170° C. and lower-side temperature was 210° C.).

The determination of bread specific volume was carried out according tothe rapeseed exclusion method. The volume and weight of the bread weremeasured after cooling at room temperature for 1h.

The specific volume (mL/g)=volume (mL)/weight (g).

The increased specific volume (%)=(the specific volume of sample−thespecific volume of blank control)/the specific volume of blankcontrol*100%

The results were shown in Table 2. It indicated that the maximumincreased specific volume of the bread adding the lipase variant was28%, while the parent lipase was 21%. It is clearly that the lipasevariants of the present invention can significantly increase thespecific volume of bread and making it a highly promising candidate forfuture applications in bread baking.

TABLE 2 Results of application of lipase in flour products The increasedThe increased white- specific volume ness compared with Enzyme (%) theblank control the parent lipase 21 1.3 mutant 1 22 1.6 mutant 2 27 1.7mutant 3 23 2.2 mutant 4 28 1.8 mutant 5 25 1.9 mutant 6 24 2.3

Example 6 Application of Lipase Variants in Steamed Bread

The experiments mainly focused on the impact of the lipase to thewhiteness of the flour products such as steamed bread. And theapplication effect was compared between the parent lipase and thevariants.

The preparation of steamed bread was carried out according to thefollowing method. The materials were original steamed bread powderwithout any improver and 0.8% yeast (Angel Yeast Co., Ltd.). The addedamount of lipase was 500 U/kg, while the blank control was zero. 40%water was added to the materials above and mixed thoroughly. The mixturewas pressed (the dough was pressed into pieces and folded and thenpressed for six times), hand-formed, proofed (proofed in the doughproofing machine at 37.5° C. with a relative humidity of 80%) and thensteamed for 20 min.

The whiteness was determined by averaging the whiteness of eight randomposition of the steamed bread.

The increased whiteness of the blank control was set as 0. Results(shown in Table 2) indicated that the maximum increased whiteness ofsteamed bread adding lipase variants was 2.3 units, while it was 1.3 inthe steamed bread adding parent lipase. Obviously, the lipase variantsin the present invention can significantly increase the whiteness offlour products such as steamed bread, which indicating a highlypromising applications in processing of flour products.

While the present invention has been described in some detail forpurposes of clarity and understanding, one skilled in the art willappreciate that various changes in form and detail can be made withoutdeparting from the true scope of the invention.

What is claimed is:
 1. A lipase mutant, comprising: an amino acidsequence with substitutions selected from the group consisting of P298T,P298T/H317P, P298T/H317P/V326S, P298T/T218S/S234F,P298T/H317P/P168UA129S, and P298T/S234F/K161R/V326S, wherein thesubstitutions are relative to a parent amino acid sequence set forth inSEQ ID NO: 1; and the lipase mutant maintains a triglyceride andlecithin hydrolysis activity and good performance in thermostability. 2.The lipase mutant of claim 1, wherein the amino acid sequence comprisesthe substitution P298T/T218S/S234F, and wherein the substitutions arerelative to the parent amino acid sequence set forth in SEQ ID NO:
 1. 3.The lipase mutant of claim 1, wherein the amino acid sequence comprisesthe substitution P298T/H317P/P168L/A129S, and wherein the substitutionsare relative to the parent amino acid sequence set forth in SEQ ID NO:1.